1. Field of the Invention
The present invention relates, in general, to a photon echo apparatus and method, and, more particularly, to an optically locked photon echo apparatus and method, which includes a nonlinear optical medium having three energy levels and an optical pulse generation unit for generating five or more optical pulses resonating between the energy levels of the nonlinear optical medium.
2. Description of the Related Art
The storage of quantum optical data is necessary for quantum information processing ranging from quantum computers that require a shorter storage time to quantum communications such as long distance quantum cryptography that requires a longer storage time.
Similarly to all classical optical memory devices, the performance of quantum memory is also determined by writing speed, bandwidth, and storage time. Even though slow light-based quantum memory which uses stopped light has succeeded in proving the storage of quantum optical data, actual limitations such as limited storage time and the inevitable use of only one piece of quantum optical data have become obstacles to quantum communications [C. Liu, Nature Vol. 409, pp. 490-493 (2001)].
On the other hand, research into photon echoes has been intensively conducted in order to apply the photon echoes to quantum memory owing to their characteristics such as high speed access rate, ultrawide bandwidth, and extremely longer storage time [G. Hetet, Phys. Rev. Lett. Vol. 100, p. 023601 (2008), and B. Hosseini, Nature Vol. 461, pp. 241-245 (2009)].
Recently, photon echoes have succeeded in experimentally demonstrating the storage of quantum optical data using a rare-earth-doped solid medium [H. de Riedmatten, Nature Vol. 456, pp. 773-777 (2008)].
To be used in quantum memory, two-pulse photon echoes must have more than 50% echo efficiency. However, in the two-pulse photon echoes, the storage time is limited by the optical phase relaxation time and is about 100 μs in a rare earth-doped optical medium.
The storage time of three-pulse photon echoes is limited by the optical population decay time and is similar to the optical phase relaxation time. Unlike the two-pulse photon echoes, three pulse photon echoes cannot retrieve more than 50% of stored information due to optical coherence loss attributable to optical population decay.
Recently, several modifications, which are intended to extend the storage time as well as to improve retrieval efficiency, have been proposed to make up for the problems with photon echoes that have occurred to date. With regard to the storage time, there is only one modification which has succeeded, that is, an Atomic Frequency Comb (AFC) method [Afzelius, M., Phys. Rev. A 79, 052329 (2009)]. AFC, however, is problematic in that a long preparation time is required because to replace two π/2-π/2 optical pulse sequences in the three-pulse photon echo system, hundreds of optical pulse trains, each composed of two weak optical pulse pair, are used to make a sharp spectral grating.
In phase-locked echo methods, the storage time is extended from optical phase relaxation time to spin dephasing time, which is inversely proportional to spin inhomogeneous broadening [B. S. Ham, Opt. Exp. Vol. 18, No. 2, pp. 1704-1713 (2010)].
Further, in the case of phase-locked photon echoes or resonant Raman echoes, optical coherence loss occurs during the storage time [B. S. Ham, Nature Photon. Vol. 3, pp. 518-522 (2009)].
Furthermore, in modified two-pulse photon echo methods using Controlled Reversible Inhomogeneous Broadening (CRIB), phase inversion is realized using an externally applied Direct Current (DC) voltage or a magnetic field gradient, where spontaneous emission noise caused by a rephasing pulse in conventional two-pulse photon echoes is eliminated. However, the storage time in CRIB is still limited by the optical phase relaxation time, which is much shorter than a millisecond [M. Nilsson and S. Kroll, Opt. Commun. Vol. 247 (No. 4-6), pp. 393-403 (2005)].